Sources of tritium

Sources of tritium production are both natural and artificial. Tritium of natural origin is generated in the upper atmosphere from atmospheric nitrogen or oxygen, by bombardment with cosmic ray neutrons or solar protons. Annual production is about 4 to 8 106 Ci resulting in a steady-state world inventory of 0.7 to 1.4 108 Ci. From this, 99% are located in the hydrosphere as HTO and 1% remains in the atmosphere as HTO vapor or HT gas. Tritium can be generated inside the Earth's crust by (n,α) reactions from aluminium and lithium silicates but this contribution remains small in comparison with the previous source. Artificial tritium comes from bomb tests performed after 1963 and from nuclear power activities. Tritium generated by reactors operation can escape both through the air and liquid effluents (tritiated water) of reactors. Tritium leakage limits are set to maximum individual doses of 0.5 rem/year. With the hypothesis of a world installed nuclear power of 2000 GW(e), the annual tritium production is estimated to 40 MCi and would represent a cumulative production of 250 MCi. With the present day rate of development of nuclear power, tritium activity in the environment would represent 108 Ci at the end of the 20. century. About 30 years would be necessary to reduce this level to the natural value. (J.S.)